The invention relates to the field of inkjet printing and more particularly to inkjet printing with additional spot colors.
Inkjet printers produce images on a receiver by ejecting ink droplets onto the receiver in an imagewise fashion. The advantages of non-impact, low-noise, low process control requirements, low energy use, and low cost operation, in addition to the capability of the printer to print on plain paper and to readily allow changing the information to be printed, are largely responsible for the wide acceptance of ink jet printers in the marketplace.
Drop-on-demand and continuous stream inkjet printers, such as thermal, piezoelectric, acoustic, or phase change wax-based printers, have at least one printhead from which droplets of ink are directed towards a recording medium. Within the printhead, the ink is contained in one or more channels. By means of power pulses, droplets of ink are expelled as required from orifices or nozzles at the end of these channels. The mechanisms whereby ink ejection works in these various types of machines are well established and will not be further discussed in the present application for letters patent.
The inkjet printhead may be incorporated into a carriage type printer, a partial width array type printer, or a pagewidth type printer. The carriage type printer typically has a relatively small printhead containing the ink channels and nozzles. The printhead can be attached to a disposable ink supply cartridge as one piece, and the combined printhead and ink cartridge assembly is attached to a carriage. In other arrangements ink is supplied on a continuous basis to the printhead via a hose arrangement from an ink reservoir located away from the inkjet printhead. The carriage is reciprocated to print one swath of information (equal to the length of a column of nozzles in the paper advance direction) at a time on a recording medium, which is typically maintained in a stationary position during the reciprocation. After the swath is printed, the paper is stepped a distance equal to the width of the printed swath or a portion thereof, so that the next printed swath is contiguous or overlapping therewith. Overlapping is often employed to address a variety of undesirable inkjet printing artifacts that may be traced, for example, to nozzle performance. This procedure is repeated until the entire page is printed.
In contrast, the pagewidth printer includes a substantially stationary printhead having a length sufficient to print across one dimension of a sheet of recording medium at a time. The recording medium is moved past the page width printhead in a direction substantially perpendicular to the printhead length. In most cases, the separation between individual nozzles is greater than the required dot spacing on the media, and hence the media may be passed under the page width printhead more than once whilst translating the printhead. By this method, printing may be done at the interstitial positions, thereby to cover the desired area of the media.
Clearly, an inkjet printer may have a printhead that extends partway across the medium to be printed upon. In such a case, the printer is known as a partial pagewidth printer. The printing medium has to be passed repeatedly under the printhead while the printhead translates laterally over a considerable distance to ensure that the appropriate area of the printing medium is ultimately addressed with ink.
While inkjet technology has found its way into the industrial environment, it has tended to be confined to specialty areas. These include printing variable data and graphics on plastic cards and tags as well as on ceramics, textiles and billboards. It is also used in the personalization of addressing for direct mail and, most importantly, in print proofing applications. The focus has clearly been on exploiting the abilities of inkjet technology as they pertain to direct digital printing of variable information and in areas where the more established printing technologies are not cost effective, such as very short run length printing jobs.
While inkjet technology has been driven strongly by consumer use of this technology, it has not yet substantially penetrated the high run length, low cost, high quality printing market. The demands and requirements of this are rather different from those of the consumer environment. In this particular industrial marketplace, the need for high throughput, quality of print and reliability at a low cost per page is particularly strong. The standards in this respect are set by other technologies such as offset A printing, gravure and flexography. Offset printing and gravure, in particular, have had the benefit of many decades and even centuries of development.
Inkjet printer technology, in contrast, is conceptually strongly based on the principles of other consumer products such as personal typewriter and the dot matrix computer printer. The typical consumer inkjet system therefore shares with the typewriter and the dot-matrix printer such aspects as stepped roller-and-carriage-based medium advance as well as replacement cartridge-based ink-media.
There is a clear need for addressing some key aspects of inkjet technology that limit the wider application of this technology in areas served by the more traditional and high throughput technologies of gravure, offset and flexography. A large body of work has been done, particularly in the case of so-called drop-on-demand inkjet printers, on making ever-higher nozzle-density inkjet printheads using ever more sophisticated technology. However, in order to make reliable industrial inkjet systems that can challenge the more established printing technologies, some of the key challenges reside elsewhere in the printer system.
In the case of an inkjet system employing state-of-the-art inkjet printheads, the ink needs to be of a type that matches the receiver media and have such properties as will keep it from clogging the inkjet nozzles. Ink supply, and the removal and management of the gas dissolved in such ink, is a subject of considerable concern in many high performance inkjet systems and many complex solutions are devoted to resolving this matter. However, these are mostly aimed at ink cartridge-based systems.
It has been demonstrated that, as long as they are supplied with de-gassed or de-aerated ink and their pulsing duty cycle is maintained at a high enough level, piezoelectric inkjet systems are quite reliable. These two issues are central to the design and manufacture of a high reliability inkjet printer aimed at competing with traditional low unit cost, high throughput printing presses. In such a system, a large number of individual printheads may be combined on an inkjet printhead assembly, numbers of sixty or more being projected. This represents a very large number of nozzles indeed, particularly in view of the increased density of inkjet nozzles on printheads used in many recent products, each nozzle having a statistical probability of failure. The two issues of duty cycle and ink de-gassing are therefore exacerbated to a great degree by this form of implementation.
Provided these two issues are adequately addressed, piezoelectric inkjet ejection systems form the preferred technological platform for such inkjet systems. Unfortunately piezoelectric inkjet heads, in particular, are very susceptible to ink ejection failure when supplied with aerated inks. This stems from the fact that they operate on the basis of creating a pressure pulse within a small body of ink. The presence of gas or air within that body of ink totally disturbs the execution of this pressure pulse. It is therefore of critical importance to ensure that an adequate supply of de-gassed ink is supplied to the nozzles at all times during printing. The general principles of de-aeration or de-gassing of inkjet ink are well-known to those skilled in the art of inkjet technology. They will therefore not be presented here again.
The second issue, being that of duty cycle, should also not be underestimated. The reliability of all inkjet systems hinges strongly on the ability of individual nozzles to produce consistently ejected droplets in repetitive fashion. Prolonged periods of non-use of a given nozzle therefore constitute an invitation to failure through the nozzle clogging with drying or dried ink. Great effort has therefore been expended in the field of inkjet technology on the matter of maintenance systems for inkjet printers. One of the primary maintenance functions is that of capping the individual printhead when it is not in use. However, it is not generally practicable to cap just a fraction of the diminutive nozzles on a given individual printhead. For this reason it is important to maintain a minimum duty cycle on any given nozzle on an individual printhead, prevention being better than cure. The entire individual printhead is then capped when not in use.
The inkjet printer therefore ejects ink as regularly as possible from each inkjet nozzle without unnecessarily wasting ink. This firing rate, combined with the large number of nozzles, creates a consumption rate of ink that exceeds by far that which may be maintained through the manual replacement of exhausted de-gassed ink containers. This adds to the requirement for ink de-gassing to occur in-line as part of the operation of the inkjet printer.
Another shortcoming of prior art inkjet printers applied to industrial printing situations, is the difficulty in handling color. High quality printing is usually not in the capability of a 4 color Cyan (C), Magenta (M), Yellow (Y), and Black (K) printer since it will not provide the color gamut required to render images in accurate color. The first steps that are usually taken to address this problem is to supplement the CMYK colors (commonly referred to as process colors) with additional colors to improve image rendition. One common scheme makes use of the standard CMYK set with additional lower concentration Magenta and Cyan in order to improve the appearance of highlights that look grainy when printed with full concentration inks. Highlights are lightest or whitest areas of a halftone reproduction, having the lowest density of dots. The addition of Orange and Green is often used to improve flesh-tones while adding the primary colors of Red, Green, and Blue also improves the color gamut of the printing device.
While the approach of using these extended color schemes works relatively well in the consumer market environment, as well as certain specific industrial applications, there is a clear need for inkjet printers to be able to print specialty colors, also known as xe2x80x9cspot colorsxe2x80x9d, on a commercially viable basis. Parties familiar with established printing technologies, such as offset lithographic printing, gravure, and flexography, appreciate that commercial printing relies on the ability to do spot colors for many aspects of printing. The printing of trademarked logos, for example, very often employs very accurately specified colors. It is very often true that the standard process colors, even if augmented with colors to increase the general color gamut as described earlier, simply cannot accurately match a particularly specified color. In commercial printing, it is usual to specially formulate a particular ink that exactly matches a logo color for printing of corporate brochures and other printing work. Furthermore, special printing effects such as fluorescent and metallic colors are not reproducible with any of the standard inksets and obviously necessitate the use of spot colors.
In published patent application, WO9634763A1 an inkjet printer that increases the number of print colors available is disclosed. This device is equipped with five or more receiving stalls so that one or more specialized or spot colors can be incorporated, in addition to the usual CMYK colors, while the speed and quality of the printing operation is not affected. The specific device embodiment shown is a carriage inkjet printer with a conventional architecture. The disclosure is specifically addressed at introducing spot colors without adversely affecting printing speed or quality. Additionally, carriage inkjet printers with as many as twelve slots for various color cartridges are now available. These printers allow the user flexibility in selecting inksets or adding spot colors.
In page-wide inkjet printers, by partially or completely dispensing with the reciprocating carriage motion, very high throughput devices can be constructed that have productivity approaching that of conventional lithographic printing systems. By nature, since these devices are intended to compete with established commercial printing techniques, it is necessary to enable the use of spot colors to provide a competitive product. Incorporating spot colors in a page-wide device represents a significant logistical challenge in that the page-wide array comprises a multiplicity of printheads of each color and adding one or more spot colors significantly increases the number of printheads. Setting up and replenishing a page-wide spot color printhead with multiple cartridges would be an extremely tedious processes and changing spot colors from job to job under these circumstances is impractical. Similarly accommodating a large number of spot colors is also impractical due to space constraints, connectivity, and other logistical considerations. Clearly, methods of dealing with the problems encountered in providing a workable spot-color handling solution for a high productivity page-wide or partial page-wide inkjet printer are lacking.
It is an objective of the present invention to provide a method and apparatus to allow page-wide inkjet printing on a commercially practicable basis employing spot colors.
The present invention is described in reference to a high-throughput inkjet printing device capable of printing process colors and optionally printing spot colors. The printing device is equipped with one or more spot color printheads that can accommodate one or more spot colors for extending the general color gamut and/or printing specially mixed spot color inks. There are less inkjet nozzles provided for each spot color than for each process color. Pages with spot colors are printed either at full resolution and a reduced printing rate, or the spot colors are printed at full print rate at reduced resolution.